Recently a friend of mine asked me a question about image sizes and how to resize them. The sizes below are what she sent me in her request, but the conversion can be made to any image.
Sizing down is easy and no loss in image quality occurs. Sizing up is a problem because you are using fewer pixels to make more.
Assume the image in question is 1124 x 768 and the resolution is 114 pixels per inch (dpi). You can see these dimensions if you open up the "Image Size" dialog box in Photoshop (see below) (Image> Image Size). Look at the top numbers in this dialog box. The inches size is irrelevant.
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By changing the pixels per inch size to 100 for inkjet printing, you can easily downsize the image with no problems. Number of pixels (top of dialog box) stays the same, while the inches dimension increases to accommodate the lower pixels/inch ratio. Notice that the file size of 2.25M also stays the same. Make sure "Resample Image" is unchecked when you do this.
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To make the file large enough to print on an offset printer, you should change the pixels/inch to 300 or whatever the printer requires. Make sure "Resample Image" is unchecked when you do this. Notice how the inches dimensions get smaller to accommodate the increase in pixels/inch and that the overall file size remains the same (2.25M).
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Now let’s convert the file to a larger file, more than 1124 x 768. But let’s keep the pixels/inch at 100. By increasing the width by 110 percent, you can get away with increasing the image dimensions and file size (see below). Do not increase at increments higher than 110%. The image will become very pixeled and look terrible!
Make sure "Scale Styles," "Constrain Proportions," and "Resample Image" are all checked. You can do this repeatedly until you get the dimensions that you are looking for.
You can't simply interpolate and upsize the file without consequences unless you do it this way.
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You can't do this with an image from the Internet unless it is larger than 72 dpi to begin with. Generally Internet images are 3 or 2 inches at 72 dpi.
I made an action in Photoshop to speed up this process. If you were to do this 30 times or so to the same image it may fall apart. Just remember: garbage in, garbage out.
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This is the increase after you followed the action above. Now the actual inches have been enlarged as well as the size of the image, which is now 2.72M.
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